Abstract: Pharmacological agents targeting receptors coupled to GTP binding proteins represent promising clinical agents spanning a multitude of human diseases. Within the G-protein coupled receptor (GPCR) superfamily, it has been most challenging to develop high throughput screening strategies to identify drugs modulating Gi/Go-linked receptors. Previous HTS methods examining these receptors have been indirect, cumbersome, and expensive, relying upon either an inhibition of drug-stimulated cAMP accumulation or the use of chimeric G-proteins. We propose to develop a screening strategy for Gi/o-linked receptors by measuring thallium flux through G protein regulated Inwardly Rectifying Potassium (K+) (GIRK) channels. Using this technique in HEK cells stably expressing GIRK 1 and 2 channels, we have generated preliminary data using muscarinic agonists and antagonists and have observed dose-dependent regulation of these channels by M2 and M4selective agents. Using a 384 well plate format, we have generated preliminary Z' values of >0.5, indicating that this assay is amenable to HTS. We plan to use this technique to perform a small scale, 8000 compound, directed HTS screen for drugs interacting with the metabotropic glutamate receptor 7 (mGluR7). This receptor, based upon its cellular location and functional activity, is known to couple to GIRK in in vitro systems and is predicted to serve as a novel target for neurological and psychiatric disorders. Finally, we will perform secondary screens to validate potential hits and determine specificity for mGluR7 versus other mGluRs. It is anticipated that these studies will open new avenues for Gi/Go-linked receptor screening as well as generate valuable tools and drug leads for mGluR7. Lay summary: G-protein coupled receptors (GPCRs) represent accessible therapeutic targets in human disease. Within the GPCR family, it has been challenging to develop technically direct, time-efficient, sensitive, and cost effective assays to identify drugs targeting receptors coupled to Gi/Go GTP binding proteins. We propose to develop and characterize a new high throughput screening technique for receptors that signal through Gi/o to regulate the G protein regulated Inwardly Rectifying Potassium (K+) channel. Using the metabotropic glutamate receptor 7 (mGluR7) as an initial prototype Gi/o-coupled receptor, we will screen a small, targeted library to develop new tools and potential lead compounds for agents modulating mGluR7, a protein for which limited pharmacological agents are available and which represents a novel drug target in neurological and psychiatric diseases such as epilepsy and schizophrenia.